Polymer matrix composites are comprised of the sub-groups of reactive matrix composites and non-reactive matrix composites. Reactive matrix composites typically produce low molecular weight volatiles during curing. Non-reactive matrix composites, also commonly referred to as thermoplastics, are generally high molecular weight structures which are heated to flow and then cooled to cure. When laying up composites prior to curing, adjacent layers may entrap air pockets in between. Additionally, during the curing process, volatiles may be released during the curing process whether as a result of a reaction, from solvent remaining in the prepeg, or otherwise.
Autoclaves have been utilized in fabrication of composite materials. In many circumstances, an autoclave provides enhanced processing flexibility compared to other common processing techniques such as ovens and presses. However, composite fabrication by autoclave is often costly in terms of labor consumption as well as capital investment. Furthermore, autoclave fabrication techniques typically limit the size of the parts which can be produced.
One technique utilized to overcome disadvantages of autoclave fabrication is single-vacuum-bag processing in an oven utilizing vacuum bag pressure. To date, this is believed to be one of the most cost effective out-of-autoclave fabrication techniques for fiber-reinforced resin matrix composites. However, this process and technique is often ineffective when a reactive resin matrix or solvent containing prepreg is present. A reactive resin (e.g., poly(amide acid)/NMP) typically generates reaction by-products (e.g., water) during curing at elevated temperatures. In order to produce a void-free quality laminate, it is often imperative to deplete these volatiles and solvents before commencing forced consolidation. The traditional single-vacuum-bag (SVB) assembly inherently hinders and/or retards the volatiles depletion mechanisms during composite fabrication because a vacuum-generated compaction force is applied to the laminate during volatile depletion. In addition, the one atmospheric pressure associated with the SVB processing tends to create excessive resin flash out of the composite during the B-stage period (i.e., the low temperature ramp-and-hold step as shown in FIG. 1). As a result, resin content and net shape of the consolidated laminate become difficult to control.
Accordingly, an improved out-of-autoclave fabrication technique for use with fiber-reinforced resin matrix composite is believed to be necessary, especially when a reactive resin matrix or solvent containing prepeg is present.
The cure cycle (temperature and pressure profiles) for manufacturing composite laminate with a reactive resin matrix such as poly(amide acid)/NMP resin system or a solvent containing prepreg usually consists of a two-step ramp-and-hold temperature profile as shown in FIG. 1. Temperature and hold duration times in each step are unique for a given composite system. The low temperature ramp-and-hold step is called the B-stage. During the B-stage, prepregs are heated and reaction by-products such as water from the resin's chemical reactions and volatiles from the solvent are generated. However, because of the absence of pressure, volatiles (i.e., solvent and reaction by-products) are free to escape.
Pressure is applied during the high temperature ramp-and-hold step (i.e., the final cure step) to afford laminate consolidation and to attain good physical properties of the resin matrix. The residual volatile level and resin fluidity remaining inside the composite are determined by these temperatures and hold duration steps. Once the consolidation pressure is applied, residual volatiles are locked in and unable to escape. In order to produce a void-fee high quality laminate, the residual processability and the temperature at which forced consolidation is commenced must be achieved through proper design of the cure cycle.
The processability of composites with reactive resin matrices involves a balance between the degree of volatile depletion and the residual fluidity remaining in the polymer. When volatiles are not depleted adequately before the forced consolidation, voids are produced, yielding a laminate with inferior quality. On the other hand, excessive cure advancement in the resin results when excessive B-stage conditions (i.e., severe temperature and prolonged time) are employed, making the composite unprocessable under moderate pressures, due to high resin viscosity. The kinetics of volatile diffusion through the liquid phase is strongly dependent upon resin chemistry, chemoviscosity, temperature and duration at the given temperature. While the reactive resin matrix continues to cure during the B-stage period, the resin fluidity continues to diminish and the composite processability suffers. In order to achieve a void-free laminate, the cure (molding) cycle must enable a sufficient percentage of volatiles to be depleted through the thermal B-stage (in the absence of pressure) before consolidation. In the meantime, an appreciable degree of residual resin fluidity should remain after the B-stage allowing infiltration of resin through fiber bundles in the composite during the pressure consolidation stage initiated at the latter stage of the cycle. Such a balancing act between the degree of residual volatiles and residual fluidity during composite fabrication is very complex and makes the design of cure cycles very challenging.
A schematic drawing to illustrate the concept of a traditional Single-Vacuum-Bag (SVB) in composite manufacturing is shown in FIG. 2. Fiber reinforced reactive resin matrix prepregs are laid up between the caul and tool steel plates. They are then enclosed by a vacuum bag, sealed around the edges onto a tool plate. A vacuum port is built through the tool plate communicating with the environment inside the bag. In the prior art, this assembly is installed in a forced air circulation oven and subjected to a cure cycle for composite curing.
When the bag is purged to atmosphere (i.e., without any vacuum), the bag rests at an equilibrium balanced by the same atmospheric pressure (i.e., 14.7 Psi) from either side of the bag in FIG. 2(a). Under this circumstance, the composite is not subjected to any external compaction forces and remains bulky and loose. During the B-stage (i.e., low temperature ramp-and-hold period in FIG. 1), the resin softens and becomes molten at elevated temperatures. Reaction by-products are generated by the resin chemical reactions and chemo-viscosity builds up as well. In order to deplete the volatiles (i.e., reaction by-products and solvent), a vacuum is pulled to accelerate this process. However, because of the pressure differential, the vacuum causes the bag to collapse tightly onto the caul plate and compact the composite at the same time as shown in FIG. 2(b). Both the tightened fibrous architecture and increasingly viscous resin matrix inside the composite, because of vacuum suction and temperature increase during B-stage, create narrower passages for volatiles to escape. Sometimes in practice, prolonged B-stage time durations are employed to lower the residual volatile levels. However, this is not always successful due to resulting poor residual matrix fluidity rendering the composite to become unprocessable.
Polymeric prepreg material is commonly impregnated with a solution of resin to provide tack and drape for handleability. The SVB assembly and process are simply too primitive, too time consuming (i.e., costly) and ineffective in removing solvent and reaction by-product during composite fabrication.